The stuff that planets are made of

October 09, 2018

Is there a second Earth out there in space? Our knowledge of planetary systems far, far away is increasing constantly, as new technologies continue to sharpen our gaze into space. To date, 3,700 planets have already been discovered outside our solar system. The planetary masses and radii of these exoplanets can be used to infer their mean density, but not their exact chemical composition and structure. The intriguing question about what these planets could look like is thus still open.

"Theoretically, we can assume various compositions, such as a world of pure water, a world of pure rock, and planets that have hydrogen-helium atmospheres and explore what radii are expected" explains Michael Lozovsky, a doctoral candidate in the group of Prof. Ravit Helled at the Institute for Computational Science at the University of Zurich.

Thresholds for planetary composition

Lozovsky and collaborators have used databases and statistical tools to characterize exoplanets and their atmospheres. These are fairly common and surrounded by a volatile layer of hydrogen and helium. However, the directly measured data previously didn't allow the researchers to determine the exact structure, since different compositions may lead to the same mass and radius. In addition to the accuracy of the data relating to mass and radius, the research team thus also investigated the assumed internal structure, temperature and reflected radiation in 83 of the 3,700 known planets, for which the masses and radii are well-determined.

"We used a statistical analysis to set limits on possible compositions. Using a database of detected exoplanets, we found that every theoretical planetary structure has a 'threshold radius', a planetary radius above which no planets of this composition exist," explains Michael Lozovsky. The amount of elements in the gaseous layer that are heavier than helium, the percentage of hydrogen and helium, as well as the distribution of elements in the atmosphere are important factors in determining the threshold radius.

Super-Earths and mini-Neptunes

The researchers from the Institute for Computational Science found that planets with a radius of up to 1.4 times that of Earth (6,371 kilometers) can be earth-like, i.e. they have a composition similar to Earth. Planets with radii above this threshold have a higher share of silicates or other light materials. Most of the planets with a radius above 1.6 radii of the Earth must have a layer of hydrogen-helium gas or water in addition to their rocky core, while those larger than 2.6 Earth radii can't be water worlds and therefore might be surrounded by an atmosphere. Planets with radii larger than 4 Earth radii are expected to be very gaseous and consist of at least 10 percent hydrogen and helium, similarly to Uranus and Neptune.

The findings of the study provide new insights into the development and diversity of these planets. One particularly interesting threshold concerns the difference between large terrestrial-like planets - otherwise known as super-Earths - and small gaseous planets, also referred to as mini-Neptunes. According to the researchers, this threshold lies at a radius of three times that of Earth. Below this threshold, it is therefore possible to find earth-like planets in the vast expanse of the galaxy.
-end-
Literature: M. Lozovsky, R. Helled, C. Dorn, and J. Venturini. Threshold Radii of Volatile-Rich Planets. Astrophysics. Astrophysical Journal. 9. October 2018, DOI: 10.3847/1538-4357/aadd09

Contacts:
Michael Lozovsky
Institute for Computational Science (ICS)
University of Zurich
Phone +41 44 635 61 89
E-mail: michloz@mail.com

University of Zurich

Related Hydrogen Articles from Brightsurf:

Solar hydrogen: let's consider the stability of photoelectrodes
As part of an international collaboration, a team at the HZB has examined the corrosion processes of high-quality BiVO4 photoelectrodes using different state-of-the-art characterisation methods.

Hydrogen vehicles might soon become the global norm
Roughly one billion cars and trucks zoom about the world's roadways.

Hydrogen economy with mass production of high-purity hydrogen from ammonia
The Korea Institute of Science and Technology (KIST) has made an announcement about the technology to extract high-purity hydrogen from ammonia and generate electric power in conjunction with a fuel cell developed by a team led by Young Suk Jo and Chang Won Yoon from the Center for Hydrogen and Fuel Cell Research.

Superconductivity: It's hydrogen's fault
Last summer, it was discovered that there are promising superconductors in a special class of materials, the so-called nickelates.

Hydrogen energy at the root of life
A team of international researchers in Germany, France and Japan is making progress on answering the question of the origin of life.

Hydrogen alarm for remote hydrogen leak detection
Tomsk Polytechnic University jointly with the University of Chemistry and Technology of Prague proposed new sensors based on widely available optical fiber to ensure accurate detection of hydrogen molecules in the air.

Preparing for the hydrogen economy
In a world first, University of Sydney researchers have found evidence of how hydrogen causes embrittlement of steels.

Hydrogen boride nanosheets: A promising material for hydrogen carrier
Researchers at Tokyo Institute of Technology, University of Tsukuba, and colleagues in Japan report a promising hydrogen carrier in the form of hydrogen boride nanosheets.

World's fastest hydrogen sensor could pave the way for clean hydrogen energy
Hydrogen is a clean and renewable energy carrier that can power vehicles, with water as the only emission.

Chemical hydrogen storage system
Hydrogen is a highly attractive, but also highly explosive energy carrier, which requires safe, lightweight and cheap storage as well as transportation systems.

Read More: Hydrogen News and Hydrogen Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.